There has not been a lack of attempts to prepare aliphatic polyisocyanates containing uretdione groups and being free as far as possible from by-products, using catalysts whose selectivity is dependent only little, if at all, on temperature and conversion.
Aliphatic isocyanates which contain uretdione groups, have a low by-product content and are based on optionally branched, linear aliphatic diisocyanates are distinguished by a particularly low viscosity; products based on cycloaliphatic diisocyanates can be used as internally blocked crosslinkers, free from elimination products, in coating systems.
Tris(dialkylamino)phosphines (DE-A 3 030 513) optionally in conjunction with cocatalysts (DE-A 3 437 635) exhibit good selectivity for the formation of uretdione groups (uretdione selectivity). Their technical usefulness is hindered, however, by the serious imperfection represented by the high carcinogenic potential of their phosphorus(V) oxide, e.g. hexamethylphosphoric triamide. DE-A 3 739 549 discloses catalytic NCO dimerization with 4-dialkylamino-pyridines, such as 4-dimethylaminopyridine (DMAP), for example, but the formation of uretdione is selective only in the case of specific cycloaliphatic isocyanates such as isophorone diisocyanate (IPDI). Linear aliphatic isocyanates such as hexamethylene diisocyanate (HDI) and also branched, linear aliphatic isocyanates such as trimethylhexane diisocyanate (TMDI) and methylpentane diisocyanate (MPDI), when used with DMAP and related compounds, give heterogeneous reaction products which are predominantly highly coloured.
DE-A 1 670 720 discloses the preparation of aliphatic polyisocyanates containing uretdione groups, in which the catalysts used are tertiary phosphines having at least one aliphatic substituent or boron trifluoride and its adducts, respectively. It is noted that high fractions of uretdione groups in the product can be obtained only at low conversions and at reaction temperatures between 50 and 80° C., with the simultaneous formation of isocyanate trimers (isocyanurates and iminooxadiazine-diones) and also, particularly at a relatively high temperature, of other by-products such as carbodiimides or uretonimines. Uretonimines are especially disruptive since they tend to give off monomeric isocyanate during storage.
In order to terminate the reaction at low conversions the phosphine catalysts are deactivated by alkylation with dimethyl sulfate (DE-A 1 670 720) or methyl toluenesulfonate (EP-A 377 177) and then unreacted monomer is removed from the product. This deactivation reaction requires temperatures of up to 60° C. and, on account of its duration, leads to a delay in the actual termination of the reaction of uretdione formation and hence, overall, to the increased formation of by-products.
According to the teaching of DE-A 19 54 093 this problem is circumvented by using elemental sulphur as terminating agent. The reaction is stopped suddenly, independently of the reaction temperature. However, the amount of sulphur required is difficult to determine, since partial catalyst deactivation occurs during the catalysed reaction. Amounts of the catalyst poison used in excess then lead to unadvantageous properties of the polyisocyanate product, such as turbidity, for example, and to problems affecting the reuse of unreacted monomer, as a result of contamination with sulphur.
It was an object of the invention, therefore, to provide a process for preparing isocyanates containing uretdione groups which as compared with the prior art exhibits a greater selectivity for uretdione formation (uretdione selectivity) in conjunction with equal or higher monomer conversions, and where at the same time there should be a distinct reduction in the propensity for uretonimines to form.